The present invention generally relates to a sampling valve useful in liquid chromatography and, in particular, relates to such a valve having a dosing bore adapted to accept an injection needle.
In a prior art sample inlet valve, for example that described in the brochure, "Operating Instructions for Model 7125 Syringe Loading Sample Injector" of Rheodyne Incorporated, Berkley, California, a stationary stator and a rotor rotatable relative thereto are provided. In use, an injection needle is inserted into the rotor to contact the surface of the stator, which needle does not enter the stator beyond the contact surface. The dosing bore in the stator having a smaller diameter than the injection needle, whereby such entry is prevented. After the sample has been delivered into the dosing bore, the rotor is rotated to change the sample inlet valve over. The rotation takes place while the injection needle remains in its position in the rotor. If the injection needle were to be withdrawn prior to the changing of the sample inlet valve, sample fluid would be sucked from the stator into the rotor via the plunger action of the injection needle. This sample quantity sucked into the rotor would cause cross-contamination.
Another difficulty encountered in prior art valves is that the changing of the rotor results in the end of the injection needle scratching the contact surface of the stator. This results in undesirable wear and necessitates the use of particularly hardened contact surfaces.
It is also necessary, in prior art valves, to utilize an injection needle which is provided with a flush end, the tip of which then sealingly engages the contact surface. Thus, in the prior art, it is not feasible to use pointed injection needles. The use of such needles would further increase the wear of the contact surface. Further, if a pointed needle were used, liquid would undoubtedly remain in the rotor-side bore, into which the injection needle is inserted. This would also result in cross-contamination.